The pesent invention relates to a mass transfer apparatus for the flow of fluids separated by a planar semipermeable membrane element folded several times upon itself and to a method of manufacturing such an apparatus.
For convenience, the invention is described hereinafter by way of example for the particular case where such an apparatus is used for the treatment of blood by hemodialysis and/or ultrafiltration; and more particularly, such an apparatus which is of relatively small size and weight, economical in operation, and especially suitable to be used at home and then discarded after use.
The invention relates to an improvement in an apparatus comprising a semipermeable membrane folded to form a stack of accordion pleats, disposed inside of a casing, and provided with the necessary ports for the introduction and evacuation of blood, dialysate and/or ultrafiltrate, the pleats of the stack on one side of the membrane containing a plurality of porous or open-mesh support members.
An apparatus of this general type is known, as discloed in U.S. Pat. No. 3,788,482. Generally the stack of the membrane in accordion pleats and of the support members is enclosed in a fluid-tight manner inside a housing. This housing may be either a box consisting of essentially two or more pieces suitably joined together for example, or an envelope made out of a suitable plastic material and molded about said stack. These boxes, being made according to a given model, have all exactly the same dimensions and, on the other hand, the mold being the same for all these envelopes, their external dimensions are all identical. Also it has been thought advantageous to have identical apparatuses which have especially stacks of exactly the same height. However, it has now been found that in actual practice the thickness of the membrane, and particularly of the support members, may vary slightly from one apparatus to another and that such slight variations, when multiplied by the number of pleats or number of support elements in a stack, may result in appreciable variations in the thickness of the blood films formed in different apparatuses of the same stack height and same external dimensions. Consequently, the performance and efficiency of these apparatuses may vary at random from one apparatus to another in an undesirable manner, presenting potentially serious problems in a field where reliability and uniformity of operation are deemed essential.
One aspect of this invention therefore lies in recognizing the nature of this problem and its cause; another aspect is concerned with the discovery of a solution to that problem. Specifically, it is an object of the present invention to provide an apparatus having reproducible performance characteristics so that the performance from one such apparatus to another will be substantially identical. It is an object of the present invention to provide an apparatus of the kind indicated that eliminates or at least greatly minimizes problems and disadvantages of the prior art devices.
It is also an object of the present invention to provide an apparatus of simple and economical construction which allows an easy flow of blood under a small pressure drop and with uniform distribution of blood over the entire membrane, and within successive pleats of the same membrane, and which requires only a limited volume of blood within the body of the apparatus.